1. Field of the Invention
This invention relates to a liquid crystal display, and more particularly to an array substrate for a liquid crystal display and a fabricating method thereof.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls a light transmittance using an electric field to display a picture. To this end, the LCD includes a liquid crystal panel having liquid crystal cells arranged in a matrix form, and a driving circuit for driving the liquid crystal panel. The liquid crystal panel is provided with pixel electrodes and a common electrode for applying electric fields to the respective liquid crystal cells. Typically, for each liquid crystal cell, the pixel electrode is provided on a lower substrate, whereas the common electrode is formed on the entire surface of an upper substrate. Each of the pixel electrodes is connected to a thin film transistor (TFT), which is used as a switching device. The pixel electrode, along with the common electrode, drives the liquid crystal cell in accordance with a data signal applied to the TFT.
Referring to FIGS. 1 and 2, an array substrate 1 of an LCD includes a TFT arranged at an intersection between a data line 13 and a gate line 11, a pixel electrode 23 connected to a drain electrode 7 of the TFT, a data pad portion DP connected to the data line 13, and a gate pad portion GP connected to the gate line 11.
The TFT includes a gate electrode 3 connected to the gate line 11, a source electrode 5 connected to the data line 13, and a drain electrode 7 connected, via a drain contact hole 19b, to the pixel electrode 23. Further, the TFT includes semiconductor layers 15 and 17, which define a channel between the source electrode 5 and the drain electrode 7 when a gate voltage is applied to the gate electrode 3. Such a TFT responds to a gate signal from the gate line 11 to selectively apply a data signal from the data line 13 to the pixel electrode 23.
The pixel electrode 23 is positioned at a cell area divided by the data line 13 and the gate line 11, and is made from a transparent conductive material having a high light transmittance. The pixel electrode 23 is electrically connected, via the drain contact hole 19b, to the drain electrode 7. The pixel electrode 23 generates a potential difference relative to a common transparent electrode (not shown) provided at an upper substrate (not shown) by a data signal applied via the TFT. By this potential difference, the liquid crystal molecules in the liquid crystal positioned between the array substrate 1 and the upper substrate are rotated due to its dielectric anisotropy. Due to this rotation, the liquid crystal allows selective transmission of light emitted from a light source through the pixel electrode 23 and the upper substrate.
The gate pad portion GP transmits a scanning signal, i.e., a gate pulse, which is generated from a gate driving integrated circuit (IC) (not shown), to the gate line 11. A gate pad terminal electrode 28 of the gate pad portion GP electrically contacts a gate pad 25 via a gate contact hole 19c. 
The data pad portion DP transmits a data signal from a data driving IC (not shown) to the data line 13. A data pad terminal electrode 29 electrically contacts a data pad 27 via a data contact hole 19a. 
Hereinafter, a method of fabricating the liquid crystal display device having the above-mentioned configuration will be described.
First, as shown in FIG. 3A, a gate metal layer is deposited on array substrate 1 of the LCD, and is patterned to form a gate pad 25 and a gate electrode 3. As shown in FIG. 3B, a gate insulating film 9 is formed on the entire surface of the array substrate 1, which has been provided with the gate pad 25 and the gate electrode 3. First and second semiconductor layers are deposited on the gate insulating film 9, and patterned to form an active layer 15 and an ohmic contact layer 17.
Subsequently, a data metal layer is deposited over the gate insulating film 9 and patterned to form a data pad 27, a source electrode 5 and a drain electrode 7, as shown in FIG. 3C. After the source electrode 5 and the drain electrode 7 are patterned, a portion of the ohmic contact layer 17 that is positioned over the gate electrode 3 is removed to expose the active layer 15. A portion of the active layer 15 corresponding to the gate electrode 3 between the source electrode 5 and the drain electrode 7 forms a channel.
Then, an insulating material is deposited over the gate insulating film 9 and patterned to form a protective layer 21, as shown in FIG. 3D. A data pad contact hole 19a and a drain contact hole 19b are formed in the protective layer 21 to expose the data pad 27 and the drain electrode 7, respectively. Also, a gate pad contact hole 19c is formed through the protective layer 21 and the gate insulating film 9 to expose the gate pad 25.
Subsequently, as shown in FIG. 3E, a transparent conductive material is deposited on the protective layer 21 and patterned to form a pixel electrode 23, a gate pad terminal electrode 28, and a data pad terminal electrode 29. The pixel electrode 23 electrically contacts the drain electrode 7 via the drain contact hole 19b. The gate pad terminal electrode 28 electrically contacts the gate pad 25 via the gate contact hole 19c, and the data pad terminal electrode 29 electrically contacts the data pad 27 via the data contact hole 19a. 
The data pad 27, the source electrode 5 and the drain electrode 7 provided on the array substrate 1 of the LCD are formed of a single layer of chrome (Cr) or molybdenum (Mo), etc., and are collectively referred to as “data metal layer.” As the trends of the LCD technology move towards finer device structures, a three-layer structure of first to third metal layers 6a, 6b and 6c (FIG. 4) has been proposed as the data metal layer. Here, the first and third metal layers 6a and 6c are made of a transparent conductive material and Mo, which is electrically stable, respectively, and the second metal layer 6b is made of aluminum (Al) or an aluminum alloy.
When such a three-layer structure of the data metal layer is patterned by a wet etching technique, the first and third metal layers 6a and 6c are likely to be ionized by an etchant liquid much more than the second metal layer 6b due to an electrode potential difference created between the first and third metal layers 6a and 6c and the second metal layer 6b. In other words, the first and third metal layers 6a and 6c are oxidized by the second metal layer 6b, and the second metal layer 6b is deoxidized by the first and third metal layers 6a and 6c. For this reason, the first and third metal layers 6a and 6c are more undercut than the second metal layer 6b, as illustrated in the circled magnified view designated by letter A in FIG. 4. When the protective layer 21 is deposited thereafter, the second metal layer 6b, which has a high reactivity, collapses. The collapsed second metal layer 6b becomes in contact with active layer 15, resulting in an increased leakage current of the TFT. Furthermore, since a deposition process of the data metal layer having such a three-layer structure requires three steps, the process is complex and incurs additional costs.
In order to overcome these problems, it has been proposed that the data metal layer be formed into a two-layer structure of first and second metal layers 6a′ and 6b′, as shown in FIG. 5. Here, the first metal layer 6a′ is made of Al or an Al alloy while the second metal layer 6b′ is made from Mo.
A method of fabricating the array substrate of the LCD having the data metal layer with such a two-layer structure includes the steps of substrate cleaning, substrate patterning, alignment film formation, annealing, substrate joining, liquid crystal injection and packaging processes. During these processes, a protective film patterning process, an annealing process, an alignment film process, and a seal-curing process are performed at temperatures greater than about 200° C. The second metal layer 6b′ of the two-layer data metal layer collapses when it undergoes a heat treatment at about 200° C. or higher, thereby causing infiltration into the active layer 15, diffusion, spark phenomena, etc. In other words, the active layer 15 becomes in contact with the second metal layer 6b′, resulting in characteristic deterioration and degradation of the TFT, such as presence of a large leakage current, etc.
In order to reduce such a leakage current, an attempt was made in that the data metal layer of two-layer structure is formed in such a sequence that the second metal layer 6b′ is formed first and the first metal layer 6a′ is formed subsequently. In this case, because the second metal layer 6b′ (bottom layer) is made from Mo, a leakage current can be somewhat suppressed. However, there occurs a disadvantage of an increased contact resistance between the first metal layer 6a′ made of aluminum and the transparent electrode formed subsequently.